Nanoindentation on diamond-like carbon and alumina coatings

A linear relationship of the square of indentation hardness and the inverse of penetration depth predicted by the mechanism-based strain gradient (MSG) theory has been used to derive the coating-only hardness of the hard coatings and the critical penetration depth, where the influence of the soft substrate comes in. The ratio of the critical penetration depth to the coating thickness is approximately 0.2 for both diamond-like-carbon (DLC) and alumina coatings and almost independent of the coating thickness. Observation of residual indents on both DLC and alumina coatings by atomic force microscopy (AFM) reveals that plastic deformation occurs in the hard coatings when the penetration depth is smaller than the critical depth. With further penetration, partial lateral cracks first initiate in the middle of the quadrants between the radial plastically deformed grooves induced by the indenter edges, then propagate to the corner of the indent and connect with one another to form an annular crack. With the increase of the penetration depth more annular cracks are created and form a growth ring of cracks. A significant delamination may occur at the centre of the indent for a deep enough penetration.